Zoom lens

ABSTRACT

A zoom lens which includes, in order from an object side, a first lens unit of positive refractive power, a second lens unit of positive refractive power and a third lens unit of negative refractive power, wherein the first, second and third lens units are moved such that, during zooming from a wide-angle end to a telephoto end, a separation between the first lens unit and the second lens unit increases and a separation between the second lens unit and the third lens unit decreases, and wherein the zoom lens has a diffractive optical element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/160,769,filed Sep. 24, 1998 now U.S. Pat. No. 6,215,600.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to zoom lenses and, more particularly, toa zoom lens having a diffractive optical element which is suited tolens-shutter cameras, video cameras, or like optical apparatuses.

2. Description of Related Art

For the lens-shutter camera or the like that does not require the longback focal distance, many zoom lenses so far proposed are of the typecomprising, in order from an object side, a first lens unit of positiverefractive power and a second lens unit of negative refractive power,wherein the air separation between these two lenses is varied to effectzooming.

Also, in Japanese Laid-Open Patent Applications No. Sho 56-128911, No.Sho 57-201213, No. Sho 60-170816, No. Sho 60-191216 (U.S. Pat. No.4,659,186), No. Sho 62-56917, etc., zoom lenses which are improved to acompact form have been proposed each comprising, in order from an objectside, a first lens unit of positive refractive power and a second lensunit of negative refractive power, wherein the separation between thesetwo lens units is varied to effect zooming. As these zoom lenses employthe plus-minus refractive power arrangement in this order from theobject side, the back focal distance is made relatively short. Moreover,the physical length for the telephoto end of the complete lens isshortened, while still maintaining realization of a high opticalperformance.

Further, as derived from the 2-unit zoom lens by dividing the first lensunit of positive refractive power into two parts of positive refractivepowers, another type of zoom lens is obtained. That is, a zoom lens isconstructed from three lens units in total and has a plus-plus-minusrefractive power arrangement, whereby the action of varying the focallength is laid on the second and third lens units to assure a greatincrease of the zoom ratio. Such a 3-unit zoom lens has been proposedin, for example, Japanese Laid-Open Patent Applications No. Hei3-282409, No. Hei 4-37810, No. Hei 4-76511, No. Hei 4-223419, No. Hei5-264903, etc.

In the meantime, for the purpose of facilitating the correction ofchromatic aberrations, it is known to provide part of an optical systemwith a diffractive optical element, as disclosed in Japanese Laid-OpenPatent Applications No. Hei 4-213421 (U.S. Pat. No. 5,044,706), No. Hei6-324262, No. Hei 9-19273 and No. Hei 9-19274.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide a compact zoom lens havinggood optical performance. In particular, a good stability of thecorrection of chromatic aberrations has to be maintained throughout theentire zooming range. It is, therefore, another object of the inventionto provide a zoom lens having a diffractive optical element ofheretofore unknown form.

To attain the above objects, in accordance with an aspect of theinvention, there is provided a zoom lens which comprises, in order froman object side, a first lens unit of positive refractive power, a secondlens unit of positive refractive power and a third lens unit of negativerefractive power, wherein the first, second and third lens units aremoved such that, during zooming from a wide-angle end to a telephotoend, a separation between the first lens unit and the second lens unitincreases and a separation between the second lens unit and the thirdlens unit decreases, and wherein the zoom lens has a diffractive opticalelement.

In accordance with another aspect of the invention, there is provided azoom lens which comprises, in order from an object side, a first lensunit of positive refractive power and a second lens unit of negativerefractive power, wherein an air separation between the first lens unitand the second lens unit is varied to effect zooming, and wherein thefirst lens unit includes at least three lenses and the first lens unithas a diffractive optical element.

In accordance with a further aspect of the invention, there is provideda zoom lens which comprises, in order from an object side to an imageside, a first lens unit of positive refractive power and a second lensunit of negative refractive power, wherein an air separation between thefirst lens unit and the second lens unit is varied to effect zooming,and wherein the first lens unit has a positive lens disposed closest tothe object side and a negative lens disposed closer to the image sidethan the positive lens, and the zoom lens has a diffractive opticalelement.

In accordance with a further aspect of the invention, there is provideda zoom lens which comprises, in order from an object side, a first lensunit of positive refractive power and a second lens unit of negativerefractive power, wherein an air separation between the first lens unitand the second lens unit is varied to effect zooming, and wherein thefirst lens unit has two positive lenses and two negative lenses and thefirst lens unit has a diffractive optical element.

In accordance with a further aspect of the invention, there is provideda zoom lens which comprises, in order from an object side, a first lensunit of positive refractive power and a second lens unit of negativerefractive power, wherein an air separation between the first lens unitand the second lens unit is varied to effect zooming, and wherein thefirst lens unit has one positive lens and two negative lenses, and thefirst lens unit has a diffractive optical element.

In accordance with a further aspect of the invention, there is provideda zoom lens which comprises, in order from an object side to an imageside, a first lens unit of positive refractive power and a second lensunit of negative refractive power, wherein an air separation between thefirst lens unit and the second lens unit is varied to effect zooming,and wherein a stop is disposed within the first lens unit and, when thefirst lens unit is divided into a front lens sub-unit closer to theobject side than the stop and a rear lens sub-unit disposed closer tothe image side than the stop, the rear lens sub-unit has a diffractiveoptical element.

These and further objects and aspects of the invention will becomeapparent from the following detailed description of preferredembodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIGS. 1A, 1B and 1C are longitudinal section views of a numericalexample 1 of the zoom lens in three operative positions.

FIGS. 2A, 2B and 2C are longitudinal section views of a numericalexample 2 of the zoom lens in three operative positions.

FIGS. 3A, 3B and 3C are longitudinal section views of a numericalexample 3 of the zoom lens in three operative positions.

FIGS. 4A, 4B, 4C and 4D are graphic representations of the aberrationsof the zoom lens of the numerical example 1 in the wide-angle end.

FIGS. 5A, 5B, 5C and 5D are graphic representations of the aberrationsof the zoom lens of the numerical example 1 in a middle focal lengthposition.

FIGS. 6A, 6B, 6C and 6D are graphic representations of the aberrationsof the zoom lens of the numerical example 1 in the telephoto end.

FIGS. 7A, 7B, 7C and 7D are graphic representations of the aberrationsof the zoom lens of the numerical example 2 in the wide-angle end.

FIGS. 8A, 8B, 8C and 8D are graphic representations of the aberrationsof the zoom lens of the numerical example 2 in a middle focal lengthposition.

FIGS. 9A, 9B, 9C and 9D are graphic representations of the aberrationsof the zoom lens of the numerical example 2 in the telephoto end.

FIGS. 10A, 10B, 10C and 10D are graphic representations of theaberrations of the zoom lens of the numerical example 3 in thewide-angle end.

FIGS. 11A, 11B, 11C and 11D are graphic representations of theaberrations of the zoom lens of the numerical example 3 in a middlefocal length position.

FIGS. 12A, 12B, 12C and 12D are graphic representations of theaberrations of the zoom lens of the numerical example 3 in the telephotoend.

FIG. 13 is a sectional view of a monolayer diffractive optical element.

FIG. 14 is a graph for explaining the wavelength-dependentcharacteristic of the diffractive optical element shown in FIG. 13.

FIG. 15 is a graph of the MTF characteristics of the zoom lens of thenumerical example 1 using the diffractive optical element shown in FIG.13.

FIG. 16 is a sectional view of a diffractive optical element of thelaminated type

FIG. 17 is a graph for explaining the wavelength-dependentcharacteristic of the diffractive optical element shown in FIG. 16.

FIG. 18 is a graph of the MTF characteristics of the zoom lens of thenumerical example 1 using the diffractive optical element shown in FIG.16.

FIG. 19 is a sectional view of a diffractive optical element of anotherlaminated type in which the two diffraction gratings are equal ingrating thickness to each other.

FIG. 20 is a longitudinal section view of a numerical example 4 of thezoom lens.

FIG. 21 is a longitudinal section view of a numerical example 5 of thezoom lens.

FIG. 22 is a longitudinal section view of a numerical example 6 of thezoom lens.

FIG. 23 is a longitudinal section view of a numerical example 7 of thezoom lens.

FIG. 24 is a longitudinal section view of a numerical example 8 of thezoom lens.

FIG. 25 is a longitudinal section view of a numerical example 9 of thezoom lens.

FIGS. 26A and 26B are graphs of the MTF characteristic of the zoom lensof the numerical example 4 using the diffractive optical element shownin FIG. 13.

FIGS. 27A and 27B are graphs of the MTF characteristic of the zoom lensof the numerical example 4 using the diffractive optical element shownin FIG. 16.

FIGS. 28A, 28B, 28C and 28D are graphic representations of theaberrations of the zoom lens of the numerical example 4 in thewide-angle end.

FIGS. 29A, 29B, 29C and 29D are graphic representations of theaberrations of the zoom lens of the numerical example 4 in a middlefocal length position.

FIGS. 30A, 30B, 30C and 30D are graphic representations of theaberrations of the zoom lens of the numerical example 4 in the telephotoend.

FIGS. 31A, 31B, 31C and 31D are graphic representations of theaberrations of the zoom lens of the numerical example 5 in thewide-angle end.

FIGS. 32A, 32B, 32C and 32D are graphic representations of theaberrations of the zoom lens of the numerical example 5 in a middlefocal length position.

FIGS. 33A, 33B, 33C and 33D are graphic representations of theaberrations of the zoom lens of the numerical example 5 in the telephotoend.

FIGS. 34A, 34B, 34C and 34D are graphic representations of theaberrations of the zoom lens of the numerical example 6 in thewide-angle end.

FIGS. 35A, 35B, 35C and 35D are graphic representations of theaberrations of the zoom lens of the numerical example 6 in a middlefocal length position.

FIGS. 36A, 36B, 36C and 36D are graphic representations of theaberrations of the zoom lens of the numerical example 6 in the telephotoend.

FIGS. 37A, 37B, 37C and 37D are graphic representations of theaberrations of the zoom lens of the numerical example 7 in thewide-angle end.

FIGS. 38A, 38B, 38C and 38D are graphic representations of theaberrations of the zoom lens of the numerical example 7 in a middlefocal length position.

FIGS. 39A, 39B, 39C and 39D are graphic representations of theaberrations of the zoom lens of the numerical example 7 in the telephotoend.

FIGS. 40A, 40B, 40C and 40D are graphic representations of theaberrations of the zoom lens of the numerical example 8 in thewide-angle end.

FIGS. 41A, 41B, 41C and 41D are graphic representations of theaberrations of the zoom lens of the numerical example 8 in a middlefocal length position.

FIGS. 42A, 42B, 42C and 42D are graphic representations of theaberrations of the zoom lens of the numerical example 8 in the telephotoend.

FIGS. 43A, 43B, 43C and 43D are graphic representations of theaberrations of the zoom lens of the numerical example 9 in thewide-angle end.

FIGS. 44A, 44B, 44C and 44D are graphic representations of theaberrations of the zoom lens of the numerical example 9 in a middlefocal length position.

FIGS. 45A, 45B, 45C and 45D are graphic representations of theaberrations of the zoom lens of the numerical example 9 in the telephotoend.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, preferred embodiments of the invention will be described indetail with reference to the drawings.

First Embodiment

FIGS. 1A to 1C through FIGS. 3A to 3C are lens block diagramsrespectively showing three numerical examples 1 to 3 of zoom lensesaccording to a first embodiment thereof (whose numerical data are to bedescribed later).

FIGS. 4A to 4D through FIGS. 6A to 6D show the aberrations of the zoomlens of the numerical example 1 in the wide-angle end, the middle focallength position and the telephoto end, respectively. FIGS. 7A to 7Dthrough FIGS. 9A to 9D show the aberrations of the zoom lens of thenumerical example 2 in the wide-angle end, the middle focal lengthposition and the telephoto end, respectively. FIGS. 10A to 10D throughFIGS. 12A to 12D show the aberrations of the zoom lens of the numericalexample 3 in the wide-angle end, the middle focal length position andthe telephoto end, respectively. In the lens block diagrams, FIGS. 1A,2A and 3A show the arrangement in the wide-angle end, FIGS. 1B, 2B and3B in the middle focal length position and FIGS. 1C, 2C and 3C in thetelephoto end.

In FIGS. 1A, 2A and 3A, a zoom lens comprises, in order from an objectside, a first lens unit L1 of positive refractive power, a second lensunit L2 of positive refractive power and a third lens unit of negativerefractive power. SP stands for a stop, and IP stands for an imageplane.

The arrows indicate the directions of movement of all the lens unitswith the loci thereof, during zooming from the wide-angle end to thetelephoto end.

The present embodiment sets forth appropriate rules of design for therefractive powers of the first to third lens units to move all the lensunits toward the object side in such relation that, during zooming fromthe wide-angle end to the telephoto end, the air separation between thefirst and second lens units increases and the air separation between thesecond and third lens units decreases.

With this arrangement of the zoom lens, the function of varying thefocal length is laid on the second and third lens units. Particularly,the third lens unit is made to contribute to a largest proportion of thevariation of the focal length. The predetermined zoom ratio is thussecured with ease. Then, provision is made for facilitating correctionof chromatic aberrations by using a diffractive optical element in thesecond lens unit or each of the second and third lens units. Thisassures maintenance of good stability of aberrations against zooming. Azoom lens of high optical performance is thus obtained.

In the zoom lenses of the numerical examples 1 to 3, the first lens unitL1 is constructed with two lenses, i.e., a negative lens 11 of meniscusform convex toward the object side and a positive lens 12 of meniscusform convex toward the object side.

The second lens unit L2 is constructed also with two lenses, i.e., anegative lens 21 of meniscus form convex toward the image side and apositive lens 22 having a convex surface facing the image side.

The third lens unit L3 is constructed with one lens, i.e., a negativelens 31 having a concave surface facing the object side.

In the zoom lenses of the numerical examples 1 and 2 shown in FIGS. 1Ato 1C and FIGS. 2A to 2C, respectively, a surface on the object side ofthe negative lens 21 in the second lens unit L2 and a surface on theobject side of the negative lens 31 in the third lens unit L3 areprovided with respective diffractive optical elements for correctingchromatic aberrations. In the zoom lens of the numerical example 3 shownin FIGS. 3A to 3C, a surface on the object side of the negative lens 31in the third lens unit L3 is provided with a diffractive optical elementfor correcting chromatic aberrations.

The stop SP is positioned on the object side of the second lens unit L2and arranged to move together with the second lens unit L2 duringzooming.

In the present embodiment, in order to minimize the bulk and size of thezoom lens, the zoom type and the construction and arrangement of theconstituent lenses are specified as described above. The variation withzooming of aberrations, especially chromatic aberrations, is correctedwell, thereby obtaining a high optical performance over the entirezooming range.

To further improve the aberration correction, the present embodimentsets forth the following additional features or conditions. It ispreferred to satisfy at least one of them.

(A) For the third lens unit, the diffractive optical element to be usedis formed to a diffraction grating of revolution symmetry with respectto the optical axis. As the phase φ(H) of the diffraction grating(diffractive optical surface) is given by the following expression:

φ(h)=(2π/λ)·(C2·h ² +C4·h ⁴ +C6·h ⁶ + . . . +Ci·h ^(i))

 where h is the height from the optical axis, λ is the wavelength, andCi is the phase coefficient for the term in the i-th degree, thefollowing condition is satisfied:

C2>0  (1)

The inequality of condition (1) represents that the diffractive opticalsurface in the third lens unit is negative in power. By satisfying thecondition (1), the longitudinal chromatic aberration mainly in thetelephoto region is corrected well.

(B) Another phase coefficient C4 cited above lies in the followingrange:

C4<0  (2)

 It is more preferred that this conditions is satisfied under the firstcondition (1). The inequality of condition (2) exhibits that thenegative power gradually weakens toward the margin of the diffractiveoptical surface. By satisfying the condition (2), the lateral chromaticaberration mainly in the wide-angle region is corrected well.

(C) The focal length Fbo of the diffractive optical surface of thediffractive optical element in the third lens unit lies within thefollowing range:

−40<Fbo/fw<−5  (3)

 where fw is the focal length in the wide-angle end of the entire lenssystem.

The inequalities of condition (3) give a range for the ratio of thefocal length of the diffractive optical surface to the focal length forthe wide-angle end of the entire lens system and have an aim chiefly toeffectively correct the variation with zooming of chromatic aberrations.Incidentally, it is more preferred on aberration correction to alter therange of the inequalities of condition (3) as follows:

−25<Fbo/fw<−8  (3a)

(D) For the second and third lens units, the diffractive opticalelements to be used are formed to diffraction gratings of revolutionsymmetry with respect to the optical axis. As the phase φn(H) of thediffraction grating (diffractive optical surface) in the n-th lens unitis given by the following expression:

φn(h)=(2π/λ)·(C2₁₃ n·h ² +C4_(—) n·h ⁴ +C6_(—) n·h ⁶+ . . . )

 where h is the height from the optical axis, λ is the wavelength, andCi_n is the phase coefficient for the term in the i-th degree of then-th lens unit, the following condition is satisfied:

C2_(—)2*C2_(—)3<0  (4)

 where * represents multiplication.

As two diffractive optical elements are used in the second and thirdlens units, the inequality of condition (4) is concerned with the signsof the powers the diffractive optical surfaces should have, indicatingthat the powers of the diffractive optical surfaces in the second andthird lens units are of different signs from each other.

In a case where the two diffractive optical elements are used in thesecond and third lens units of opposite refractive powers, therefore,the powers of both diffractive optical surfaces strengthen each other,so that both lens units suppress each other's chromatic aberrations.Good correction of the aberrations is thus achieved.

The zoom lenses of the numerical examples 1 and 2 satisfy all theconditions (A) to (D). The zoom lens of the numerical example 3satisfies the conditions (A) to (C).

By the way, in the present embodiment, the diffraction optical elementto be used is constructed in the kinoform shown in FIG. 13. Another typehaving two diffraction gratings of different thicknesses from each other(or, of the same thickness) stacked as shown in FIG. 16, i.e., thelaminated type, is also applicable.

FIG. 14 shows the wavelength-dependent characteristics of thediffraction efficiency for the first-order diffracted rays in thediffractive optical element 101 shown in FIG. 13. The actual structureof construction of the diffractive optical element 101 is a layer ofultra-violet ray setting resin on the surface of a substrate 102, inwhich a diffraction grating 103 is formed to such a thickness “d” thatthe diffraction efficiency of the first-order diffracted rays becomes100% at a wavelength of 530 μm.

As is apparent from FIG. 14, the diffraction efficiency in the designorder decreases as the wavelength goes away from an optimized value of530 μm. Meanwhile, in the neighborhood of the design order, i.e., in thezero and second orders, the diffraction efficiency of the diffractedrays increases. Such an increase of the diffracted rays in the otherorders than the design one causes production of flare and leads to lowerthe resolving power of the optical system.

FIG. 15 shows another characteristic of the zoom lens of the numericalexample 1 when using the diffractive optical element in the form of thegrating of FIG. 13, where the MTF (Modulation Transfer Function) isplotted versus the spatial frequency. As is apparent from FIG. 15, theMTF characteristic slightly drops in the low frequency region.

When the laminated type of diffractive optical element composed of twodiffraction gratings 104 and 105 shown in FIG. 16 is used, thewavelength-dependent characteristic of the diffraction efficiency forthe first-order diffracted rays is shown in FIG. 17.

In FIG. 16, a first diffraction grating 104 made of an ultravioletsetting resin (Nd=1.499, νd=54) is formed on a substrate 102. Then, asecond diffraction grating 105 made of another ultraviolet setting resin(Nd=1.598, νd=28) is formed on the first diifraction grating 104. Inthis combination of materials, the thickness “d1” of the firstdiffraction grating 104 is taken at d1=13.8 μm, and the thickness “d2”of the second diffraction grating 105 is taken at d2=10.5 μm.

As is understandable from FIG. 17, the making of the diffractive opticalelement in the laminated structure increases the diffraction efficiencyfor the design order to higher than 95% over the entire range of usefulwavelengths.

FIG. 18 shows the spatial-frequency-response MTF characteristics of thezoom lens of the numerical example 1 with the diffractive opticalelement in the grating form shown in FIG. 16. By using the diffractiveoptical element of the laminated structure, the MTF characteristic isimproved in the low frequency region. The desired MTF characteristic isthus obtained. It will be appreciated from the foregoing that, if, asthe diffractive optical element according to the invention, thelaminated structure is used, further improvements of the opticalperformance can be achieved.

It should be noted that, for the diffractive optical element of thelaminated structure described above, the materials are not limited tothe ultraviolet setting resin. Other materials such as plastics also maybe used instead. For certain substrates, the first diffraction grating104 may be formed directly therein. Furthermore, there is no need todifferentiate the thicknesses of the two gratings from each other. Insome combinations of materials, the thicknesses of the two gratings 104and 105 may be made equal to each other as shown in FIG. 19.

Since, in this case, no grooves are exposed to the outside from thesurface of the diffractive optical element, the dust proof is excellent,contributing to an increase of the productivity on the assembling linein manufacturing the diffractive optical elements.

Next, numerical data for the numerical examples 1 to 3 are shown intables below, where Ri is the radius of curvature of the i-th lenssurface, when counted from the object side, Di is the i-th axial lensthickness or air separation, when counted from the object side, and Niand νi are respectively the refractive index and Abbe number of theglass of the i-th lens element, when counted from the object side. Also,the values of the factors in the above-described conditions (1) to (4)for the numerical examples 1 to 3 are listed in Table-1.

The shape of an a spheric surface is expressed in the coordinates withan X axis in the axial direction and a Y axis in a directionperpendicular to the optical axis, the direction in which light advancesbeing taken as positive, by the following equation:$X = \quad {\frac{\left( {1/R} \right)Y^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right)\left( {Y/R} \right)^{2}}}} + {BY}^{4} + {CY}^{6} + {DY}^{8} + {EY}^{10} + {FY}^{12}}$

where R is the radius of the osculating sphere, and K, B, C, D, E and Fare the a spheric coefficients. Also, the notation “e-0X” means“10^(−x)”.

NUMERICAL EXAMPLE 1

f = 28.91˜111.95 Fno = 5.75˜9.7 2ω = 73.6˜21.9 R1 = 61.538 D1 = 1.20 N1= 1.721506 ν1 = 29.2 R2 = 35.290 D2 = 0.50 R3 = 12.675 D3 = 3.30 N2 =1.503779 ν2 = 66.8 R4 = 37.287 D4 = Variable R5 = Stop D5 = 2.14 R6 =−8.441 D6 = 1.00 N3 = 1.743197 ν3 = 49.3 R7 = −13.638 D7 = 0.50 R8 =186.232 D8 = 3.13 N4 = 1.583126 ν4 = 59.4 R9 = −10.221 D9 = Variable R10= −13.234 D10 = 2.00 N5 = 1.568728 ν5 = 63.2 R11 = 153.574 VariableFocal Length Separation 28.91 70.00 111.95 D4 2.35 10.66 13.65 D9 13.314.02 1.00 Aspheric Coefficients: R1: K = 0 A = 0 B = −6.871e-06 C =−3.769e-08 D = 5.046e-10 E = −3.552e-12 R9: K = 0 A = 0 B = 1.140e-04 C= 9.259e-07 D = −2.750e-08 E = 5.520e-10 R11: K = 0 A = 0 B = −5.609e-05C = 3.163e-07 D = −1.264e-09 E = 1.930e-12 Phase Coefficients: R6: C2 =−1.20630e-03 C4 = 9.34880e-06 C6 = −2.84960e-07 R10: C2 = 1.29480e-03 C4= −1.49190e-05 C6 = 7.63570e-08

NUMERICAL EXAMPLE 2

f = 38.92˜125.94 Fno = 5.75˜8.00 2ω = 58.1˜19.5 R1 = 72.122 D1 = 1.20 N1= 1.901354 ν1 = 31.6 R2 = 39.271 D2 = 0.50 R3 = 13.476 D3 = 4.75 N2 =1.487489 ν2 = 70.2 R4 = 75.872 D4 = Variable R5 = Stop D5 = 3.76 R6 =−8.441 D6 = 1.00 N3 = 1.754998 ν3 = 52.3 R7 = −12.549 D7 = 0.50 R8 =−63.218 D8 = 2.87 N4 = 1.583126 ν4 = 59.4 R9 = −10.468 D9 = Variable R10= −13.234 D10 = 2.00 N5 = 1.522493 ν5 = 59.8 R11 = 429.825 VariableFocal Length Separation 38.92 80.04 125.94 D4 1.65 8.29 11.22 D9 13.544.67 1.00 Aspheric Coefficients: R1: K = 0 A = 0 B = −5.149e-06 C =−5.861e-08 D = 1.156e-09 E = −1.044e-11 R9: K = 0 A = 0 B = 7.123e-05 C= 1.339e-06 D = −5.465e-08 E = 1.235e-09 R11: K = 0 A = 0 B = −4.342e-05C = 1.892e-07 D = −1.067e-09 E = 2.132e-12 Phase Coefficients: R6: C2 =−1.03030e-03 C4 = 1.07690e-05 C6 = −4.16160e-07 R10: C2 = 1.21780e-03 C4= −9.19150e-06 C6 = 4.83000e-08

NUMERICAL EXAMPLE 3

f = 38.94˜111.48 Fno = 5.75˜9.7 2ω = 58.1˜22.0 R1 = 56.337 D1 = 1.20 N1= 1.688930 ν1 = 31.1 R2 = 30.703 D2 = 0.50 R3 = 13.646 D3 = 4.00 N2 =1.518205 ν2 = 65.0 R4 = 45.065 D4 = Variable R5 = Stop D5 = 2.61 R6 =−9.756 D6 = 1.00 N3 = 1.721506 ν3 = 29.2 R7 = −14.475 D7 = 0.50 R8 =18833.420 D8 = 3.99 N4 = 1.583126 ν4 = 59.4 R9 = −12.523 D9 = VariableR10 = −13.234 D10 = 2.00 N5 = 1.487489 ν5 = 70.2 R11 = 549.527 VariableFocal length Separation 38.94 70.04 111.48 D4 3.21 10.15 14.26 D9 13.745.55 1.00 Aspheric Coefficients: R1: K = 0 A = 0 B = −7.935e-06 C =−2.966e-08 D = 1.419e-10 E = −9.292e-14 R9: K = 0 A = 0 B = 6.156e-05 C= 1.290e-06 D = −7.837e-08 E = 1.767e-09 R11: K = 0 A = 0 B = −5.510e-05C = 2.850e-07 D = −1.938e-09 E = 4.422e-12 Phase Coefficients: R10: C2 =6.35410e-04 C4 = −7.70360e-06 C6 = 3.07160e-08

TABLE 1 Numerical Example Condition 1 2 3 (1) C2   1.29 × 10⁻³   1.22 ×10⁻³   6.35 × 10⁻⁴ (2) C4 −1.49 × 10⁻⁵ −9.19 × 10⁻⁶ −7.70 × 10⁻⁶ (3)Fbo/fw −13.36 −10.55 −20.21 (4) C2_2*C2_3 −1.56 × 10⁻⁶ −1.25 × 10⁻⁶ —

As described above, the present embodiment (the numerical examples 1 to3) makes up a zoom lens from three lens units in total. With thisconfiguration, it sets forth appropriate rules of design for the formand the construction and arrangement of the constituent lenses in eachlens unit and makes use of a diffractive optical element or elements ofappropriate form or forms to correct well the variation of chromaticaberrations with zooming. It is, therefore, made possible to achieve azoom lens having a field angle of 60 to 70 degrees or thereabout at thewide-angle end and a zoom ratio of about 3 to 4, which has a goodoptical performance throughout the entire zooming range, while stillpermitting the total length of the entire lens system to be shortened toa compact form.

Second Embodiment

Another embodiment of the invention is next described in which a zoomlens is made up from two lens units and takes an even more compact form.

FIG. 20 to FIG. 25 are lens block diagrams of six numerical examples 4to 9 of zoom lenses in the wide-angle end. Their numerical data will bedescribed later. The zoom lens comprises, in order from an object side,a first lens unit L1 of positive refractive power and a second lens unitL2 of negative refractive power. During zooming from the wide-angle endto the telephoto end, the first and second lens units L1 and L2 axiallymove toward the object side, while decreasing the separationtherebetween. Incidentally, SP stands for a stop and IP stands for animage plane.

In the present embodiment, the use of such a zoom type has, despite thecompact form, to correct chromatic aberrations well, thus making itpossible to realize a high optical performance.

The second lens unit L2, when in the telephoto end, greatly enlarges theimage magnifying power of the first lens unit L1. To extend the range ofimage magnifications as much desired, therefore, chromatic aberrationsbecome difficult to correct. Also, to improve the compact form, therefractive power of each of the lens units L1 and L2 has to strengthen,causing an increase of the difficulty of correcting the chromaticaberrations. In order to correct the chromatic aberrations wellthroughout the entire zooming range, it becomes important to find outproper rules of design for the construction and arrangement of theconstituent lenses of each lens unit L1, L2 and to effectively use thediffractive optical element or elements.

In the zoom lens of the numerical example 4 shown in FIG. 20, the firstlens unit L1 is constructed with at least three lenses, concretelyspeaking, four lenses in a plus-minus-minus-plus refractive powerarrangement in this order from the object side. At least one diffractiveoptical element is used in the entire lens system. The chromaticaberrations are thus made possible to correct well. In actual practice,the seventh and eleventh surfaces are selected to be made as diffractiveoptical surfaces.

In a case where the diffractive optical element is used in the firstlens unit L1, the longitudinal chromatic aberration in particular can becorrected well. In another case where the diffractive optical element isused in the second lens unit L2, the lateral chromatic aberration inparticular can be corrected well.

In the zoom lens of the numerical example 5 shown in FIG. 21, the firstlens unit L1 is constructed with at least three lenses, concretelyspeaking, four lenses in a plus-minus-minus-plus refractive arrangementin this order from the object side. In addition, at least onediffractive optical element is used in each of the first and second lensunits L1 and L2, thereby making it possible to correct chromaticaberrations well. To maintain good stability of correction of thelongitudinal chromatic aberration throughout the entire zooming range,each lens unit L1, L2 must be corrected sufficiently in itself.

For the first lens unit L1, the diffractive optical element is soarranged as to correct the longitudinal chromatic aberration well initself. With this arrangement, particularly during zooming, thevariation with zooming of longitudinal chromatic aberrations issuppressed to a minimum. For the second lens unit L2, since theoff-axial light beam travels at a far side from the optical axis, thediffractive optical element used therein can correct the lateralchromatic aberration in particular. In actual practice, the eighth andthirteenth surfaces are selected to be made as diffractive opticalsurfaces.

In the zoom lens of the numerical example 6 shown in FIG. 22, the firstlens unit L1 has at least a positive lens and a negative lens,concretely speaking, three lenses in a plus-minus-plus refractive powerarrangement in this order from the object side. In the first lens unitL1, there is used at least one diffractive optical element. In actualpractice, the seventh and eleventh surfaces are made as diffractiveoptical surfaces.

The second lens unit L2 has a negative lens. Longitudinal chromaticaberration can be corrected by using a glass material of high or lowdispersion in this negative lens, or by using the diffractive opticalelement to impart a similar effect thereto. However, lateral chromaticaberration in particular tends to be difficult to correct. Nonetheless,a positive lens disposed at the position closest to the object side canbe used in correcting the longitudinal and lateral chromatic aberrationsin good balance.

Such a framework of the first lens unit L1 having at least a positivelens and a negative lens deserves an even higher optical performance. Tothis purpose, letting the focal length of the first lens unit L1 bedenoted by f1 and the focal length, the refractive index and the Abbenumber of the material of the positive lens closest to the object sidein the first lens unit L1 be denoted by fG1, ndG1 and νdG1,respectively, it is preferable to satisfy the following conditions:

0<f1/fG1<0.8  (5)

1.48<ndG1<1.70  (6)

30<νdG1<65  (7)

The inequalities of condition (5) are concerned with the focal length ofthe positive lens closest to the object side in the first lens unit L1and the focal length of the first lens unit L1 and have an aim tocorrect various aberrations. When the lower limit of the condition (5)is exceeded, the refractive power of the first lens unit L1 becomesnegative and the above-described effects cannot be obtained. So, thisviolation is objectionable. When the upper limit is exceeded, as thismeans that the refractive power of the first lens unit L1 is too strong,particularly for the wide-angle end, distortion deterioratesobjectionably. For more desired results, it is preferred to alter thelower limit of the condition (5) to “0.1” and the upper limit to “0.7”.

The inequalities of condition (6) give a range for the refractive indexof the material of the positive lens closest to the object side in thefirst lens unit L1. When the lower limit of the condition (6) isexceeded, the required curvatures for the appropriately determinedrefractive power of the surfaces of this lens tend to become tough. So,comatic aberration deteriorates objectionably. When the upper limit isexceeded, the Petzval sum increases in the negative sense, deterioratingthe focal surface characteristics objectionably. For more desiredresults, it is preferred to alter the lower limit of the condition (6)to “1.50” and the upper limit to “1.65”.

The inequalities of condition (7) give a range for the Abbe number ofthe material of the positive lens closest to the object side in thefirst lens unit L1. When the lower limit of the condition (7) isexceeded, as this means that the dispersion is too large, the difficultyof correcting the longitudinal chromatic aberration increasesobjectionably. When the upper limit is exceeded, as this means that thedispersion is too small, it becomes difficult to correct the lateralchromatic aberration well over the entire zooming range. So, thisviolation is objectionable. For more desired results, it is preferred toalter the lower limit of the condition (7) to “35” and the upper limitto “60”.

In the zoom lens of the numerical example 7 shown in FIG. 23, the firstlens unit L1 has at least two positive lenses and at least one negativelens, concretely speaking, three lenses in a plus-minus-plus refractivepower arrangement in this order from the object side. In the entire lenssystem, there is used at least one diffractive optical element. Inactual practice, the sixth and eighth surfaces are made as diffractiveoptical surfaces.

To assure improvements of the compact form, there is need to strengthenthe refractive power of each of the lens units L1 and L2. In this case,however, the optical performance is hardly retained good. Particularly,the spherical aberration and field curvature the first lens unit L1produces become difficult to correct. So, the first lens unit L1 isconstructed with inclusion of at least two positive lenses over whichthe positive refractive power is distributed. Good correction ofspherical aberration is thus made easier. At the same time, the fieldcurvature is corrected. Also, the air separation created between the twopositive lenses is usable to correct distortion particularly for thewide-angle end.

To attain a high optical performance, likewise as in the numericalexample 7, it is desirable to construct the first lens unit L1 in theform of a triplet comprising, in order from the object side, a positivelens, a negative lens and a positive lens.

Further, to correct chromatic aberrations well, it is desirable that, ofthe positive lenses in the first lens unit L1, the one which is locatedclosest to the image side and has a strongest positive refractive powerin the first lens unit L1 is selected for application of a diffractiveoptical surface. As the first lens unit L1 is positive in power,longitudinal chromatic aberration originates in large part from thatpositive lens. By providing that positive lens with the diffractiveoptical element having a negative dispersion, therefore, the chromaticaberrations the first lens unit L1 produces can be correctedadvantageously.

In the zoom lens of the numerical example 8 shown in FIG. 24, the firstlens unit L1 has at least one positive lens and at least two negativelenses, concretely speaking, four lenses in a plus-minus-minus-plusrefractive power arrangement. The optical system as a whole makes use ofat least one diffractive optical element. In actual practice, the ninthand thirteenth surfaces are made as diffractive optical surfaces.

To assure improvements of the compact form, there is need to strengthenthe refractive power of each of the lens units L1 and L2. However,because of this, the high optical performance is difficult to preserve.Particularly, the spherical aberration the first lens unit L1 producesbecomes difficult to correct. So, the first lens unit L1 is constructedwith inclusion of at least two negative lenses over which the negativerefractive power is distributed. The one of the negative lenses whichlies relatively closer to the object side can be used to correct thespherical aberration in particular.

Further, to attain a high optical performance, it is desirable that, asin the numerical example 8, the first lens unit L1 comprises, in orderfrom the object side, a positive lens, a negative lens, a negative lensand a positive lens.

Alternatively, to correct spherical aberrations well, it is desirable tomake use of at least one a spheric surface in the negative lens in thefirst lens unit L1. Since the spherical aberration the first lens unitL1 produces becomes under-corrected, the a spheric surface to be used inthe negative lens is desirably formed to such a shape that the negativerefractive power becomes progressively stronger toward the margin.

In the numerical example 9 shown in FIG. 25, the stop SP is positionedwithin the first lens unit L1. When the first lens unit L1 is dividedinto a front lens sub-unit 1 a disposed closer to the object side thanthe stop SP and a rear lens sub-unit 1 b disposed closer to the imageside than the stop SP, the rear lens sub-unit 1 b is provided with atleast one diffractive optical element. Concretely speaking, the ninthand thirteenth surfaces are made as diffractive optical surfaces.

This zoom type makes the shortest distance from the first lens unit L1to the final plane in the telephoto end. The positioning of the stop SPwithin the first lens unit L1 allows the second lens unit L2 to approachthe first lens unit L1 more closely. In turn, the refractive power ofthe second lens unit L2 can be made relatively weak to thereby producean advantage on the aberrational problem.

Further, to attain a high optical performance, letting the focal lengthsof the front and rear lens sub-units 1 a and 1 b be denoted by f(1a) andf(1b), respectively, it is preferable to satisfy the followingcondition:

f(1b)/f(1a)<0.4  (8)

When the upper limit of the condition (8) is exceeded, the refractivepower of the front lens sub-unit 1 a becomes too much stronger than thatof the rear lens sub-unit 1 b. This is objectionable. Also, if the powerof the front lens sub-unit 1 a increases in the negative sense, thediameter of the first lens unit L1 in particular increasesobjectionably. If the power of the front lens sub-unit 1 a increases inthe positive sense, the difficulty of correcting distortion particularlyfor the wide-angle end increases objectionably. Further, to attain ahigh optical performance, it is desirable to alter the upper limit ofthe condition (8) to “0.2”.

Furthermore, in all of the numerical examples 4 to 9, it is preferred tosatisfy at least one of the following features or conditions.

(a) The second lens unit L2 has a positive lens and a negative lens inthis order from the object side. This arrangement is favorable formaking it possible to correct distortion particularly for the wide-angleend. With the negative lens alone, particularly for the wide-angle end,the second lens unit L2 produces too large distortion. So, there aresome cases where the first lens unit L1 cannot correct such distortion.

To attain more improved results, it is preferred to apply an a sphericsurface to at least one surface in the second lens unit L2. As this aspheric surface is formed to a shape that the positive refractive powerbecomes progressively stronger toward the margin, particularly for thewide-angle end, it becomes possible to correct coma and distortion.

(b) For the diffractive optical element in the first lens unit L1, asthe phase φ(H) of the diffractive optical surface of the diffractiveoptical element is defined as

φ(h)=(2π/λ)·(ΣCi·h ^(i))

 where h is the height from the optical axis, λ is the wavelength, andCi is the phase coefficient in the term of the i-th degree, thefollowing condition is satisfied:

−0.1<C2<0  (9)

The inequalities of condition (9) give a range for the phase coefficientin the term of the second degree of the diffractive optical surface inthe first lens unit L1 and have an aim chiefly to correct longitudinalchromatic aberration. When the lower limit of the condition (9) isexceeded, the longitudinal chromatic aberration the first lens unit L1alone produces becomes over-corrected. The longitudinal chromaticaberration is, therefore, hardly maintained stable over the entirezooming range. So, the violation is objectionable. When the upper limitis exceeded, or when the coefficient has a positive value, as this meansthat the refractive power of the diffractive optical surface becomesnegative, the first lens unit L1 of positive refractive power cannotcorrect longitudinal chromatic aberration in itself.

(c) For the diffractive optical element in the second lens unit L2, asthe phase φ(H) of the diffractive optical surface of the diffractiveoptical element is defined as

φ(h)=(2π/λ)·(ΣCi·h ^(i))

 where h is the height from the optical axis, λ is the wavelength, andCi is the phase coefficient in the term of the i-th degree, thefollowing condition is satisfied:

0<C2<0.1  (10)

The inequalities of condition (10) give a range for the phasecoefficient in the term of the second degree of the diffractive opticalsurface in the second lens unit L2 and have an aim chiefly to correctlongitudinal chromatic aberration. When the lower limit of the condition(9) is exceeded, the longitudinal chromatic aberration the second lensunit L2 alone produces becomes over-corrected. The longitudinalchromatic aberration is, therefore, hardly maintained stable over theentire zooming range. So, the violation is objectionable. When the lowerlimit is exceeded, or when the coefficient has a negative value, as thismeans that the refractive power of the diffractive optical surfacebecomes positive, the second lens unit L2 of negative refractive powercannot correct longitudinal chromatic aberration in itself.

(d) Letting the focal lengths of the first and second lens units L1 andL2 be denoted by f1 and f2, respectively, and the focal length of theentire lens system in the wide-angle end be denoted by fw, the followingconditions are satisfied:

0.4<f1/fw<0.9  (11)

0.4<|f2/fw|<0.9  (12)

The inequalities of conditions (11) and (12) give proper ranges for therefractive powers of both lens units L1 and L2 and have an aim toimprove the compact form of the optical system in such a manner as tomaintain the good optical performance. When the lower limit of thecondition (11) is exceeded, as this means that the refractive power ofthe first lens unit L1 is too strong, distortion particularly for thewide-angle end and spherical aberration particularly for the telephotoend become difficult to correct. So, the violation is objectionable.When the upper limit is exceeded, as this means that the refractivepower of the first lens unit L1 is too weak, the total length of theentire lens system increases objectionably. To take a better balancebetween the optical performance and the improvement of the compact form,it is desired to alter the lower limit of the condition (11) to “0.5”and the upper limit to “0.8”.

When the lower limit of the condition (12) is exceeded, as this meansthat the refractive power of the second lens unit L2 is too strong,particularly for the wide-angle end, curvature of field and distortionbecome difficult to correct. When the upper limit is exceeded, as thismeans that the refractive power of the second lens unit L2 is too weak,the zooming movement of the second lens unit L2 increases largely,which, in turn, causes the total length for the telephoto end of theentire lens system to increase objectionably. Further, to take a betterbalance between the optical performance and the improvement of thecompact form, it is desired to alter the lower limit of the condition(12) to “0.5” and the upper limit to “0.75”.

(e) Each of the positive lens disposed closest to the image side in thefirst lens unit L1 and the negative lens disposed closest to the imageside in the second lens unit L2 has at least a diffractive opticalsurface. The use of such a diffractive optical surface makes it possibleto correct chromatic aberrations well.

The positive lens of strongest refractive power in the first lens unitL1 is rather better arranged at the position closest to the image sidein the first lens unit L1, so that the rear principal point of the firstlens unit L1 is put closer to the image side. In the telephoto end,therefore, the interval between the principal points of the first andsecond lens units L1 and L2 can be shortened. This produces an advantageof making it easier to determine the refractive power of each lens unitL1, L2. Since this positive lens is relatively strong in refractivepower, the longitudinal chromatic aberration of the first lens unit L1alone is caused to deteriorate. Although this longitudinal chromaticaberration can be corrected by making smaller the dispersion of thematerial of the positive lens, the availability of such a material waslimited in the past. However, it is now preferred that the diffractiveoptical element having a negative dispersion is applied to this positivelens with an advantage of effectively correcting the chromaticaberrations of the first lens unit L1. Also, the negative lens disposedclosest to the image side in the second lens unit L2 has the off-axiallight beam travelling a far point from the optical axis. It isrecommended to apply the diffractive optical element to this negativelens, so that lateral and longitudinal chromatic aberrations can becorrected at once.

(f) The grating in the diffractive optical surface is formed to anoptical system of the laminated type.

For the optical system using the diffractive optical element, to realizeachievement of a high optical performance, it is preferred that thegroove form of the diffractive optical element is made to be adiffraction grating of the laminated structure.

The diffraction grating of the diffractive optical element has akinoform shown in FIG. 13. FIG. 14 shows the wavelength-dependentcharacteristics of the diffraction efficiency for the first-orderdiffracted rays of the diffractive optical element of FIG. 13. Theactual diffractive optical element is constructed with a layer ofultra-violet ray setting resin on the surface of the substrate 102described before in which a diffraction grating 103 is formed to such athickness “d” that the diffraction efficiency of the first-orderdiffracted rays becomes 100% at a wavelength of 530 μm.

FIGS. 26A and 26B show the MTF characteristics relative to the spatialfrequency of the zoom lens of the numerical example 4 using thediffraction optical element in the form of the grating shown in FIG. 13.It is understandable that the MTF of the low frequency region slightlydrops from the desired value.

FIGS. 27A and 27B show the MTF characteristics relative to the spatialfrequency of the zoom lens of the numerical example 4 using thediffraction optical element in the form of the grating shown in FIG. 16.By using the laminated type of diffraction grating, the opticalperformance can be further improved. Incidentally, the laminated type ofdiffraction grating shown in FIG. 19 may be also used.

Next, numerical data for the six numerical examples 4 to 9 are shown intables, where Ri is the radius of curvature of the i-th lens surface,when counted from the object side, Di is the i-th axial lens thicknessor air separation, when counted from the object side, and Ni and νi arerespectively the refractive index and Abbe number of the glass of thei-th lens element, when counted from the object side.

The shape of an a spheric surface is expressed in the coordinates withan X axis in the axial direction and a Y axis in a directionperpendicular to the optical axis, the direction in which light advancesbeing taken as positive, by the following equation:$X = {\frac{\left( {1/R} \right)Y^{2}}{1 + \sqrt{1 - \left( {Y/R} \right)^{2}}} + {AY}^{2} + {BY}^{4} + {CY}^{6} + {DY}^{8} + {EY}^{10} + {FY}^{12} + {GY}^{14}}$

where R is the radius of the osculating sphere, and A, B, C, D, E, F andG are the a spheric coefficients.

The shape of the diffractive optical surface is expressed by thefollowing equation for the phase φ(h):

φ(h)=(2π/λ)·(C2·h ² +C4·h ⁴ +C6·h ⁶ +C8·h ⁸)

where h is the height from the optical axis, λ is the wavelength, and Ciis the phase coefficient in the term of the i-th degree.

NUMERICAL EXAMPLE 4

f = 28.8˜81.6 Fno = 3.8˜10.8 2ω = 61.9˜23.9  R1 = 16.531 D1 = 1.56 N1 =1.517417 ν1 = 52.4  R2 = 38.386 D2 = 1.02  R3 = −23.795 D3 = 1.44 N2 =1.834000 ν2 = 37.2  R4 = −66.900 D4 = 2.10 *R5 = −24.802 D5 = 1.20 N3 =1.583060 ν3 = 30.2  R6 = −35.241 D6 = 2.35 *R7 = 41.325 D7 = 2.56 N4 =1.487490 ν4 = 70.2  R8 = −11.265 D8 = 0.72  R9 = Stop D9 = Variable *R10= −45.826 D10 = 2.24 N5 = 1.491710 ν5 = 57.4 *R11 = −22.018 D11 = 4.22 R12 = −8.722 D12 = 1.08 N6 = 1.772499 ν6 = 49.6  R13 = −36.477 VariableFocal Length Separation 28.80 49.48 81.60 D9 9.52 4.10 1.12 AsphericCoefficients: R5: A = 0.00000 · 10⁰ B = −1.34789 · 10⁻⁴ C = −1.58360 ·10⁻⁶ D = −1.78761 · 10⁻⁸ E = 1.10032 · 10⁻⁹ F = −6.72520 · 10⁻¹² R7: A =0 B = −2.85904 · 10⁻⁵ C = −1.95252 · 10⁻⁷ D = −1.66452 · 10⁻⁸ R10: A =0.00000 · 10⁰ B = 6.86886 · 10⁻⁵ C = 1.20454 · 10⁻⁶ D = 5.42222 · 10⁻⁸ E= −1.17587 · 10⁻⁹ F = 9.44691 · 10⁻¹² R11: A = 0 B = 2.80650 · 10⁻⁶ C =−1.85905 · 10⁻⁷ D = 1.34134 · 10⁻⁹ Phase Coefficients: R7: C2 = −9.57314· 10⁻⁴ C4 = 8.95657 · 10⁻⁶ C6 = 1.03754 · 10⁻⁷ R11: C2 = 1.51659 · 10⁻³C4 = −2.8692 · 10⁻⁵ C6 = 3.77816 · 10⁻⁷

NUMERICAL EXAMPLE 5

f = 28.8˜81.6 Fno = 3.5˜9.9 2ω = 61.8˜23.9  R1 = 16.236 D1 = 1.68 N1 =1.517417 ν1 = 52.4  R2 = 35.596 D2 = 1.02  R3 = −23.855 D3 = 1.60 N2 =1.834000 ν2 = 37.2  R4 = −47.753 D4 = 3.20 *R5 = −23.700 D5 = 1.20 N3 =1.743997 ν3 = 44.8  R6 = −37.169 D6 = 1.52  R7 = 42.526 D7 = 2.80 N4 =1.487490 ν4 = 70.2 *R8 = −11.012 D8 = 0.00  R9 = Stop D9 = Variable *R10= −73.628 D10 = 2.08 N5 = 1.730770 ν5 = 40.6  R11 = −30.863 D11 = 4.11 R12 = −8.921 D12 = 1.08 N6 = 1.772499 ν6 = 49.6 *R13 = −46.845 VariableFocal Length Separation 28.81 49.46 81.59 D9 9.43 3.99 1.00 AsphericCoefficients: R5: A = 0.00000 · 10⁰ B = −1.55663 · 10⁻⁴ C = −1.36097 ·10⁻⁶ D = −2.79056 · 10⁻⁸ E = F = −9.84857 · 10⁻¹² −6.72819 · 10⁻¹² R8: A= 0 B = −8.47132 · 10⁻⁶ C = 6.78587 · 10⁻⁷ D = −1.21289 · 10⁻⁸ R10: A =0.00000 · 10⁰ B = 8.65536 · 10⁻⁵ C = 5.82389 · 10⁻⁸ D = 3.33795 · 10⁻⁸ E= F = 5.86015 · 10⁻¹² −6.24444 · 10⁻¹⁰ R13: A = 0 B = 1.07415 · 10⁻⁵ C =−7.88153 · 10⁻⁸ D = 5.47962 · 10⁻¹⁰ Phase Coefficients: R8: C2 =−1.00395 · 10⁻³ C4 = 2.66549 · 10⁻⁶ C6 = C8 = 1.45162 · 10⁻⁹ −1.18927 ·10⁻⁷ R13: C2 = 1.72371 · 10⁻³ C4 = C6 = 7.10899 · 10⁻⁸ −1.35624 · 10⁻⁵

NUMERICAL EXAMPLE 6

f = 25.0˜47.0 Fno = 4.5˜6.2 2ω = 69.2˜40.3  R1 = 9.880 D1 = 1.30 N1 =1.575006 ν1 = 41.5  R2 = 16.403 D2 = 1.02  R3 = −15.098 D3 = 1.60 N2 =1.846659 ν2 = 23.8  R4 = −24.633 D4 = 1.10  R5 = Stop D5 = 2.33 *R6 =33.634 D6 = 2.30 N3 = 1.583126 ν3 = 59.4 *R7 = −16.927 D7 = Variable  R8= −24.197 D8 = 2.10 N4 = 1.669100 ν4 = 55.4 *R9 = −14.863 D9 = 3.29  R10= −8.013 D10 = 1.10 N5 = 1.772499 ν5 = 49.6 *R11 = −35.734 VariableFocal Length Separation 25.00 31.78 47.00 D7 6.79 4.02 0.72 AsphericCoefficients: R6: A = 0.00000 · 10⁰ B = −1.11939 · 10⁻⁴ C = 1.46479 ·10⁻⁶ D = −1.79522 · 10⁻⁷ E = 3.23810 · 10⁻⁹ R9: A = 0 B = −9.86627 ·10⁻⁵ C = −9.93546 · 10⁻⁸ D = −7.26351 · 10⁻⁸ E = 7.59055 · 10⁻¹⁰ F =5.8794 · 10⁻¹² G = −2.3202 · 10⁻¹³ Phase Coefficients: R7: C2 = −1.23959· 10⁻³ C4 = 8.3075 · 10⁻⁶ C6 = 2.53174 · 10⁻⁷ C8 = −3.30477 · 10⁻⁹ R11:C2 = 1.69075 · 10⁻³ C4 = C6 = C8 = 3.69246 · 10⁻¹⁰ −1.19685 · 10⁻⁶−4.88669 · 10⁻⁸

NUMERICAL EXAMPLE 7

f = 25.0˜47.0 Fno = 4.5˜6.0 2ω = 69.2˜40.3  R1 = 9.749 D1 = 1.30 N1 =1.517417 ν1 = 52.4  R2 = 18.049 D2 = 1.08  R3 = −16.066 D3 = 1.80 N2 =1.846659 ν2 = 23.8  R4 = −35.089 D4 = 0.90  R5 = Stop D5 = 2.07 *R6 =30.522 D6 = 2.50 N3 = 1.583126 ν3 = 59.4  R7 = −14.766 D7 = Variable *R8= −24.108 D8 = 2.00 N4 = 1.669100 ν4 = 55.4 *R9 = −15.738 D9 = 3.72  R10= −7.830 D10 = 1.10 N5 = 1.712995 ν5 = 53.8  R11 = −39.932 VariableFocal Length Separation 25.00 31.76 47.00 D7 6.79 4.03 0.72 AsphericCoefficients: R6: A = 0 B = −1.2764 · 10⁻⁴ C = 8.94366 · 10⁻⁷ D =−1.40401 · 10⁻⁷ E = 1.45866 · 10⁻⁹ R9: A = 0 B = −6.61907 · 10⁻⁵ C =−1.12234 · 10⁻⁶ D = −5.76607 · 10⁻⁸ E = 7.54810 · 10⁻¹⁰ F = 3.04643 ·10⁻¹² G = −1.74614 · 10⁻¹³ R8: A = 0 B = C = −7.32508 · 10⁻¹¹ −3.82939 ·10⁻¹³ D = −1.51103 · 10⁻¹⁵ E = −7.37501 · 10⁻¹⁶ Phase Coefficients: R6:C2 = −4.57377 · 10⁻⁴ C4 = 1.01456 · 10⁻⁵ C6 = 6.53778 · 10⁻⁷ C8 =−3.45252 · 10⁻⁸ R8: C2 = 6.76246 · 10⁻⁴ C4 = C6 = 1.18510 · 10⁻⁶ C8 =−1.86918 · 10⁻⁸ −3.02685 · 10⁻⁵

NUMERICAL EXAMPLE 8

f = 24.0˜46.0 Fno = 3.5˜6.2 2ω = 71.4˜41.1  R1 = 11.183 D1 = 1.20 N1 =1.517417 ν1 = 52.4  R2 = 15.038 D2 = 0.79  R3 = −55.298 D3 = 0.95 N2 =1.677900 ν2 = 55.3 *R4 = 190.585 D4 = 1.00  R5 = Stop D5 = 1.72  R6 =42.751 D6 = 0.90 N3 = 1.846660 ν3 = 23.8  R7 = 18.983 D7 = 0.91  R8 =23.562 D8 = 2.70 N4 = 1.603112 ν4 = 60.7 *R9 = −10.171 D9 = Variable R10 = −23.997 D10 = 2.00 N5 = 1.647689 ν5 = 33.8 *R11 = −15.388 D11 =3.65  R12 = −8.141 D12 = 1.10 N6 = 1.712995 ν6 = 53.9 *R13 = −62.473Variable Focal Length Separation 24.00 30.70 46.00 D9 6.59 3.95 0.80Aspheric Coefficients: R4: A = 0.00000 · 10⁰ B = 3.03785 · 10⁻⁴ C =−4.96896 · 10⁻⁸ D = 2.17463 · 10⁻⁷ R9: A = −2.50723 · 10⁻⁸ B = 1.70205 ·10⁻¹⁰ C = 3.51108 · 10⁻¹³ D = 6.47271 · 10⁻¹⁶ R11: A = 0.00000 · 10⁰ B =−8.68396 · 10⁻⁵ C = 4.77609 · 10⁻⁸ D = −3.01109 · 10⁻⁶ E = 3.56443 ·10⁻¹¹ R13: A = −1.75606 · 10⁻⁸ B = 1.30383 · 10⁻¹⁰ C = 8.18777 · 10⁻¹³Phase Coefficients: R9: C2 = −6.16106 · 10⁻⁴ C4 = C6 = 3.16771 · 10⁻⁷ C8= −8.71789 · 10⁻⁹ −3.44559 · 10⁻⁶ R13: C2 = 8.83247 · 10⁻⁴ C4 = C6 =5.11044 · 10⁻⁸ −9.06727 · 10⁻⁶

NUMERICAL EXAMPLE 9

f = 23.0˜52.0 Fno = 4.0˜7.0 2ω = 73.8˜36.7  R1 = 11.290 D1 = 1.30 N1 =1.575006 ν1 = 41.5  R2 = 13.656 D2 = 0.82  R3 = −37.730 D3 = 0.95 N2 =1.743997 ν2 = 44.8 *R4 = 265.542 D4 = 1.30  R5 = Stop D5 = 1.58  R6 =32.042 D6 = 0.90 N3 = 1.846660 ν3 = 23.8  R7 = 18.930 D7 = 0.57  R8 =21.956 D8 = 2.70 N4 = 1.572501 ν4 = 57.8 *R9 = −9.471 D9 = Variable  R10= −20.573 D10 = 2.00 N5 = 1.730770 ν5 = 40.6 *R11 = −13.576 D11 = 3.27 R12 = −8.141 D12 = 1.10 N6 = 1.772499 ν6 = 49.6 *R13 = −52.484 VariableFocal Length Separation 23.00 31.95 52.00 D9 7.43 4.06 0.72 AsphericCoefficients: R4: A = 0.00000 · 10⁰ B = 2.55577 · 10⁻⁴ C = 6.37879 ·10⁻⁶ D = 1.17157 · 10⁻⁸ R9: A = −2.50723 · 10⁻⁸ B = 6.53118 · 10⁻⁵ C =−1.52688 · 10⁻⁶ D = 4.85868 · 10⁻⁸ R11: A = 0.00000 · 10⁰ B = −5.79810 ·10⁻⁵ C = −1.14126 · 10⁻⁶ D = −2.68089 · 10⁻⁸ E = 2.53796 · 10⁻¹⁰ R13: A= −1.75606 · 10⁻⁸ B = −2.67453 · 10⁻⁵ C = 7.40356 · 10⁻⁷ D = −5.12038 ·10⁻⁹ Phase Coefficients: R9: C2 = −1.09512 · 10⁻³ C4 = C6 = 4.78071·10⁻⁷ −1.17823 · 10⁻⁵ R13: C2 = 1.62227 · 10⁻³ C4 = C6 = 1.82451 · 10⁻⁸C8 = 3.69589 · 10⁻¹⁰ −1.02612 · 10⁻⁵

FIGS. 28A to 28D through FIGS. 30A to 30D graphically show theaberrations of the zoom lens of the numerical example 4 in thewide-angle end, the middle focal length position and the telephoto end,respectively. FIGS. 31A to 31D through FIGS. 33A to 33D graphically showthe aberrations of the zoom lens of the numerical example 5 in thewide-angle end, the middle focal length position and the telephoto end,respectively. FIGS. 34A to 34D through FIGS. 36A to 36D graphically showthe aberrations of the zoom lens of the numerical example 6 in thewide-angle end, the middle focal length position and the telephoto end,respectively. FIGS. 37A to 37D through FIGS. 39A to 39D graphically showthe aberrations of the zoom lens of the numerical example 7 in thewide-angle end, the middle focal length position and the telephoto end,respectively. FIGS. 40A to 40D through FIGS. 42A to 42D graphically showthe aberrations of the zoom lens of the numerical example 8 in thewide-angle end, the middle focal length position and the telephoto end,respectively. FIGS. 43A to 43D through FIGS. 45A to 45D graphically showthe aberrations of the zoom lens of the numerical example 9 in thewide-angle end, the middle focal length position and the telephoto end,respectively.

The values of the factors in the above-described conditions (5) to (12)for the numerical examples 4 to 9 are listed in a table below.

Condition Numerical Example No. 4 5 6 7 8 9 (5) 0.38 0.37 0.45 0.47 0.220.18 (6) 1.517 1.517 1.575 1.517 1.517 1.575 (7) 52.4 52.4 41.5 52.452.4 41.5 (8) — — 0.10 0.00 0.03 0.15 (9) −9.57* −10.0* −12.4* −4.57*−6.16* −11.0* (10)  15.2* 17.2* 16.9* 6.76* 8.83* 16.2* (11)  0.73 0.720.73 0.73 0.71 0.73 (12)  0.62 0.62 0.71 0.71 0.71 0.72 * = .10⁻⁴

As described above, the present embodiment (numerical examples 4 to 9)constructs the zoom lens in the form of the 2-unit type comprising afirst lens unit of positive refractive power and a second lens unit ofnegative refractive power with inclusion of the diffractive opticalelement or elements as effectively used to improve the compact form ofthe entire lens system while still permitting a high optical performanceto be maintained stable over the entire zooming range.

What is claimed is:
 1. A zoom lens system comprising a plurality ofmovable lens units which consist of, in order from an object side to animage side, a first lens unit of positive refractive power, and a secondlens unit of negative refractive power, wherein a separation betweensaid first lens unit and said second lens unit is varied to effectzooming, and wherein said first lens unit includes at least three lensesand said zoom lens system has a diffractive part.
 2. A zoom lens systemaccording to claim 1, wherein said second lens unit includes, in orderfrom the object side, a positive lens and a negative lens.
 3. A zoomlens system according to claim 1, wherein said first lens unit has saiddiffractive part, and wherein said diffractive part included in saidfirst lens unit is composed of a diffraction grating of revolutionsymmetry with respect to an optical axis, and, as a phrase φ(h) of saiddiffraction grating included in said first lens unit is defined asφ(h)=(2π/λ)·(ΣCi·h ^(i))  where h is a height from the optical axis, λis a wavelength, and Ci is a phase coefficient for a term in the i-thdegree, said zoom lens system satisfies the following condition:−0.1<C2<0.
 4. A zoom lens system according to claims 1, wherein saidsecond lens unit has said diffractive part, and wherein said diffractivepart included in said second lens unit is composed of a diffractiongrating of revolution symmetry with respect to an optical axis, and, asa phase φ(h) of said diffraction grating included in said second lensunit is defined as φ(h)=(2π/λ)·(ΣCi·h ^(i))  where h is a height fromthe optical axis, λ is a wavelength, and Ci is a phase coefficient for aterm in the i-th degree, said zoom lens system satisfies the followingcondition:  0<C2<0.1.
 5. A zoom lens system according to claim 1,satisfying the following conditions: 0.4<f1/fw<0.9 0.4<|f2/fw|<0.9 where f1 and f2 are focal lengths of said first lens unit and saidsecond lens unit, respectively, and fw is a focal length of said zoomlens system in a wide angle end.
 6. A zoom lens system according toclaim 1, wherein each of a positive lens disposed closest to the imageside in said first lens unit and a negative lens disposed closest to theimage side in said second lens unit has said diactive part.
 7. A zoomlens system according to claim 1, wherein said diffractive partcomprises a first diffraction rating and a second diffraction grating,wherein said first diffraction grating and said second diffractiongrating are made of different materials from each other.
 8. A zoom lenssystem according to claim 1, wherein each of said first lens unit andsaid lens unit has said diffractive part.
 9. A zoom lens systemcomprising a plurality of movable lens units which consists of, in orderfrom an object side to an image side, a first lens unit of positiverefractive power and a second lens unit of negative refractive power,wherein a separation between said first lens unit and said second lensunit is varied to effect zooming, and wherein said first lens unit hastwo positive lenses and two negative lenses and said zoom lens systemhas a diffractive part.
 10. A zoom lens system according to claim 9,wherein said first lens unit comprises in order from the object side, apositive lens, a negative lens and a positive lens.
 11. A zoom lenssystem according to claim 9, wherein said second lens unit comprises, inorder from the object side, a positive lens and a negative lens.
 12. Azoom lens system according to claim 9, wherein said first lens unit hassaid diffractive part, and wherein said diffractive part included insaid first lens unit is composed of a diffraction grating of revolutionsymmetry with respect to an optical axis, and, as a phase φ(h) of saiddiffraction grating is defined as φ(h)=(2π/λ)·(ΣCi·h ^(i))  where h is aheight from the optical axis, λ is a wavelength, and Ci is a phasecoefficient for a term in the i-th degree, said zoom lens systemsatisfies the following condition: −0.1<C2<0.
 13. A zoom lens systemaccording to claim 9, wherein said second lens unit has said diffractivepart, and wherein said diffractive part included in said second lensunit is composed of a diffraction grating of revolution symmetry withrespect to an optical axis, and, as a phase φ(h) of said diffractiongrating is defined as  φ(h)=(2π/λ)·(ΣCi·h ^(i))  where h is a heightfrom the optical axis, λ is a wavelength, and Ci is a phase coefficientfor a term in the i-th degree, said zoom lens system satisfies thefollowing condition: 0<C2<0.1.
 14. A zoom lens system according to claim9, satisfying the following conditions: 0.4<f1/fw<0.9 0.4<|f2/fw|<0.9 where f1 and f2 are focal lengths of said first lens unit and saidsecond lens unit, respectively, and fw is a focal length of said zoomlens system in a wide angle end.
 15. A zoom lens system according toclaim 9, wherein each of a positive lens disposed closest to the imageside in said first lens unit and a negative lens disposed closest to theimage side in said second lens unit has said diffractive part.
 16. Azoom lens system according to claim 9, wherein said diffractive partcomprises a first diffraction grating and a second diffraction grating,wherein said first diffraction grating said second diffraction gratingare made of different materials from each other.
 17. A zoom lens systemcomprising a plurality of lens units which consist of, in order from anobject side to an image side, a first lens unit of positive refractivepower and a second lens unit of negative refractive power, wherein aseparation between said first lens unit and said second lens unit isvaried to effect zooming, and wherein said first lens unit has onepositive lens and two negative lenses, and said zoom lens system has adiffractive part.
 18. A zoom lens system according to claim 17, whereinsaid first lens unit comprises, in order from the object side, apositive lens, a negative lens, a negative lens and a positive lens. 19.A zoom lens system according to claim 17, wherein second lens unitcomprises, in order from the object side, a positive lens and a negativelens.
 20. A zoom lens system according to claim 17, wherein said firstlens unit has said diffractive part, and wherein said diffractive partincluded in said first lens unit is composed of a diffraction grating ofrevolution symmetry with respect to an optical axis, and, as a phaseφ(h) of said diffraction grating is defined as φ(h)=(2π/λ)·(ΣCi·h ^(i)) where h is a height from the optical axis, λ is a wavelength, and Ci isa phase coefficient for a term in the i-th degree, said zoom lens systemsatisfies the following condition: −0.1<C2<0.
 21. A zoom lens systemaccording to claim 17, wherein said second lens unit has saiddiffractive part, and wherein said diffractive part included in saidsecond lens unit is composed of a diffraction grating of revolutionsymmetry with respect to an optical axis, and, as a phase φ(h) of saiddiffraction grating included in said second lens unit is defined asφ(h)=(2π/λ)·(ΣCi·h ^(i)) where h is a height from the optical axis, λ isa wavelength, and Ci is a phase coefficient for a term in the i-thdegree, said zoom lens system satisfies the following condition:0<C2<0.1.
 22. A zoom lens system according to claim 17, satisfying thefollowing conditions: 0.4<f1/fw<0.9 0.4<|f2/fw|<0.9  where f1 and f2 arefocal lengths of said first lens unit and said second lens unit,respectively, and fw is a focal length of said zoom lens system in awide angle end.
 23. A zoom lens system according to claim 17, whereineach of a positive lens disposed closest to the image side in said firstlens unit and a negative lens disposed closest to the image side in saidsecond lens unit has said diffractive part.
 24. A zoom lens systemaccording to claim 17, wherein said diffractive part comprises a firstdiffraction grating and a second diffraction grating, wherein said firstdiffraction grating and said second diffraction grating are made ofdifferent materials from each other.